1. Field of the Invention
The present invention relates in general to electrochromic devices, and, more particularly, to a multi-cell electrochromic device having more than one electrochromic device sub-assembly configured in optical alignment.
2. Background Art
Electrochromic devices have been known in the art for several years. Furthermore, the utilization of two electrochromic devices in optical alignment has been disclosed in U.S. Pat. No. 5,076,673, entitled “Prolonged Coloration Electrochromic Assembly,” which is hereby incorporated herein by reference in its entirety (hereinafter sometimes referred to as the '673 patent). However, to the best of Applicant's knowledge the '673 patent discloses electrochromic devices which are configured to color alternately, that is one electrochromic device colors while the second electrochromic device is cleared. The optically aligned devices are configured in such a way that light impacting the front surface of the device passes through both electrochromic media. While the above-identified patent teaches that a plurality of electrochromic devices, or device cells, can be configured to reduce color segregation in an electrochromic device that is intentionally colored for prolonged periods of time, the '673 patent is void of any adequate disclosure relative to the beneficial effects associated with simultaneously coloring and clearing a multi-cell electrochromic device having more than one electrochromic device sub-assembly.
By way of background, it will be understood that in a traditionally configured single compartment, solution phase, electrochromic device, which consists of two planar electrodes having an electrochromic medium contained between the two electrodes, the clearing time of the electrochromic device increases as the cell spacing or distance between the two planar electrodes increases. It will be further understood that, conversely, the current required to maintain a colored state with transmission below the highest transmission of the electrochromic device decreases as the cell spacing increases. For relatively small area electrochromic devices, such as automotive mirrors, the conductivity of transparent conductive coatings is sufficiently high to allow for the manufacture of commercially acceptable electrochromic devices—(i.e. the electrochromic devices color and clear (or cycle) in sufficiently short times because the cell spacing is relatively small, less than approximately 0.3 mm, while the electrochromic device uniformly colors and clears.
By way of contrast, however, in large area electrochromic devices, such as architectural windows or windows for transportation vehicles, if the same cell spacing is used the resistive losses associated with the conductive coatings can lead to a potential in the center of the large area electrochromic device that is less than the potential at the edge of the large area electrochromic device (e.g. near the electrical contact point) which can facilitate problematic, non-uniform coloration of such a large area electrochromic device.
One method used to overcome such a problem is to increase the cell spacing between the two traditional electrodes while, at the same time, decreasing the concentration of electrochromic materials in the medium. While such a solution leads to large area devices that color uniformly, the time required to clear the device can be quite long, which for some applications of electrochromic windows is problematic.
The concentration of electrochromic materials in the medium as is taught in U.S. Pat. No. 6,137,620, entitled “Electrochromic Media With Concentration-Enhanced Stability, Process For The Preparation Thereof And Use In Electrochromic Devices,” which is hereby incorporated herein by reference in its entirety (hereinafter sometimes referred to as the '620 patent) also impacts the steady state current of a single compartment, solution phase, electrochromic device, as well as the ultimate coloration the device can achieve. In some applications the device has a desired low-end light transmission of less than approximately 0.5% requiring a larger concentration of electrochromic materials in the medium. Devices that are able to obtain these very dark low end transmission levels are also perceived as being quite slow to clear.